With different work machines such as self-propelled work machines in the form of dump trucks, trucks, bulldozers or similar construction vehicles or mining vehicles or surface cutters such as surface miners, asphalt cutters or snow blowers, electric motors have increasingly been used in recent times for the traction drive and/or for the main work unit since such electric drives have considerable advantages over hydrostatic drives, in particular a better efficiency and an easier servicing, for instance. The higher purchasing costs of electric motors can be compensated again very fast due to the considerably better efficiency and the hereby lower operating costs.
The size of the electric motor is in this respect determined to a large extent by the starting torque or by the maximum required drive torque so that the starting torque has a great influence on the costs of the electric machine. For example, a dump truck or truck for starling up on heavy ground or on inclines requires a relative high starting torque which is considerably larger than the torque required in normal drive operation. Bulldozers, for example, require an increased drive torque when an immobile rock or a frozen dump of earth has to be pushed away. With work machines such as the named surface miners or asphalt cutters, a relatively high start-up torque can likewise be required, for example in order to release a milling drum with a jerk after it has frozen.
Although these starting or start-up situations only make up a small part of the total operating time of the respective work machine, the drive apparatus must nevertheless be designed for this in order to satisfy the operating requirements.
It has already been proposed in this respect, cf. DE 10 2010 014 644 A1 to operate the electric motor for the start-up process of the milling drum of a surface miner with a frequency inverter to limit the starting current on the ramping up and to bridge the frequency inverter after the ramp-up procedure to reduce the losses arising in the frequency inverter and the reduction of the efficiency of the electric motors in started-up operation which accompanies the operation at the frequency inverter. Such a bridging of the frequency inverter is, however, difficult when larger speed variations arise in the intended operation such as is the case in the traction drive of a truck or dump truck, for example.
U.S. Pat. No. 7,950,481 proposes a bulldozer with an electric drive whose drive torque can be briefly increased by switching in a battery, i.e. an increased power supply; however, this in turn requires a sufficiently large electric motor.
It is the underlying object of the present invention to provide an improved work machine of the initially named kind as well as an improved method for operating such a work machine to avoid the disadvantages of the prior art and to further develop the latter in an advantageous manner. In particular, the drive torque which is required for starting, which may be very high, should be able to be provided by the drive without this having to be paid for by efficiency losses in the normal operation range and by over-dimensioned, expensive electric motors.
This object is achieved in accordance with the invention by a work machine having a traction drive unit and/or a main work unit which is driven via a drive apparatus comprising at least one electric motor, wherein the drive apparatus has at least one hydraulic motor supporting the electric motor; and by a method for operating a work machine having a drive apparatus having at least one electric motor and at least one hydraulic motor for driving a traction drive unit and/or work unit, wherein the hydraulic motor is switched in for starting the traction drive unit and/or work unit and is decoupled on an exceeding of a predefined speed.
It is therefore proposed also to provide a hydraulic drive in addition to the electric drive, the hydraulic drive being able to support the electric motor as required to be able to provide the required, increased torque in special operating situations such as the starting up under unfavorable conditions, in accordance with the invention, the drive apparatus comprises at least one hydraulic motor supporting the electric motor. An additional drive torque can be provided by the hydraulic motor, which allows the electric motor to be dimensioned smaller than would be the case if the total drive torque of the traction drive or of the main work unit had to be applied by the electric motor alone.
In an advantageous further development of the invention, the hydraulic motor can be switched in by means of a clutch and can be separated from the drive train such that the hydraulic motor is only used or only has to be operated when the additional torque of the hydraulic motor is actually required, in particular in operating situations which exceed the starting torque of the electric motor. The hydraulic motor in particular does not have to be coupled in when the drive torque of the electric motor is alone sufficient such that the efficiency of the drive apparatus is considerably improved. On the other hand, the drive apparatus can be operated at higher speeds which the hydraulic motor would as a rule not tolerate, but are provided as intended by the at least one electric motor.
The named clutch by means of which the hydraulic motor can be coupled to and decoupled from the drive train can in this respect be configured as a dog clutch. Such dog clutches work largely free of loss in the opened state so that in normal operation with a decoupled hydraulic motor no losses in or impairments of the efficiency arise due to the clutch. Alternatively to such a dog clutch, however, a multi-plate clutch can also be used by means of which the hydraulic motor can advantageously also be switched in on speed differences between the electric motor and the hydraulic motor.
In a further development of the invention, the at least one electric motor and the hydraulic motor can be connected to a common drive train which conducts the drive torques at the output side from the electric motor and the hydraulic motor into the unit part to be driven. In an advantageous further development of the invention, in this respect the hydraulic motor can be connected to the electric motor in serial connection such that, with a closed clutch, the drive torque of the hydraulic motor is conducted onward via the electric motor to the output-side end of the drive train. The hydraulic motor can in particular be connected to the electric motor by means of the named clutch, with the electric motor being connectable to the hydraulic motor via the clutch, on the one hand, and being connected to the drive train, on the other hand, to drive the element or unit part to be driven.
It would, however, generally also be possible to provide the hydraulic motor and the electric motor in a parallel connection with one another and only to merge the drive torques directly at the unit part to be driven or at a drive train part connected thereto such that the drive torque of the hydraulic motor is not conducted via the electric motor. The aforesaid design with a connection of the hydraulic motor to the electric motor, however, has advantages with respect to a simple design of the drive train and to a compact arrangement.
The hydraulic motor and the electric motor can in particular be arranged coaxially with one another so that their drive axles are oriented substantially in alignment with one another. The electric motor can advantageously comprise two shaft connection stubs at oppositely disposed sides of which one is connected to the element to be driven and the other is connectable to the hydraulic motor, optionally in each case with the interposition of suitable transmission or drive elements such as a clutch, transmission, step-up or step-down stages and the like.
In an advantageous further development of the invention, the named drive train can comprise a transmission to make the achievable drive speeds of the electric motor and/or of the hydraulic motor compatible with the required speed at the drive unit part to be driven or also compatible with one another. In accordance with an advantageous further development of the invention, the named transmission can be provided between the electric motor and the output-end of the drive train. The connection of the hydraulic motor to the electric motor can be configured as transmission-free, with the clutch for coupling and decoupling the hydraulic motor naturally being able to be provided in the aforesaid manner. In this case, the hydraulic motor and the electric motor run at the same speed with a closed clutch.
Alternatively or additionally to the aforesaid design, the drive train can, however, also have a transmission between the hydraulic motor and the electric drive. The additional drive torque of the hydraulic motor can hereby be switched in over a larger speed range of the electric motor so that the additional torque of the hydraulic motor can be provided over a larger speed range.
The switching in of the hydraulic motor can generally take place in various manners, with it being taken into consideration in accordance with a simple design of the invention to control the switching in and optionally also the switching off of the hydraulic motor manually, for example, by actuating an adjustment element by which the aforesaid clutch can be closed and opened. Depending on the starting situation, the machine operator can select whether the hydraulic motor should be switched in, for example such that on a normal start on firm, level ground or with only a limited load of a dump truck or truck, a start is only made using the electric motor and the hydraulic motor remains decoupled, while the machine operator can switch the hydraulic motor in for starting on an incline and/or on deep ground or with a high load.
In a further development of the invention, however, at least one semiautomatic or automatic control of the switching in and/or switching off of the hydraulic motor can advantageously be provided, in particular such that the control apparatus for switching in and/or switching off the hydraulic motor takes account of the speed of the drive apparatus and/or the required drive torque.
For example, in an advantageous further development of the invention, the control apparatus can switch off the hydraulic motor as soon as a predefined speed is reached or exceeded and/or when the required drive torque falls below a predefined value. The named control apparatus can in particular take account of the travel speed with a self-propelled work machine such as a dump truck or truck and can decouple the hydraulic motor automatically when the travel speed exceeds a predefined value. If the hydraulic support of the electric drive is utilized for the driving of a main work unit such as the milling drum of a surface miner, of a snow blower or of an asphalt cutter, the control apparatus can decouple the hydraulic motor when the milling drum has reached a specific speed. Alternatively or additionally, the control apparatus can switch off the hydraulic motor on a falling below of a predefined drive torque which can be monitored, for example, by a suitable sensor system, for example in the form of an ammeter, which measure the current take-up of the electric motor or by another suitable power take-up sensor system which can determine the power take-up of the electric motor. The power take-up can in particular be calculated by means of a used inverter.
In a further development of the invention, the named control apparatus can optionally also control the switching in of the hydraulic motor and for this purpose in turn take account of the speed of the drive apparatus and/or the drive torque required by it, with the hydraulic motor in particular being able to be switched in when the drive speed falls below a predefined speed and/or the required drive torque exceeds a predefined value. The limit values for the drive speed or the required drive torque at which a switching in of the hydraulic motor takes place in this respect do not have to coincide with the previously named limit values for the switching off of the hydraulic motor. For example, the drive speed on whose falling below a switching in of the hydraulic motor takes place can be considerably lower than the travel speed at which the hydraulic motor is switched off. If, for example, a dog clutch is used, the control apparatus can only switch in the hydraulic motor when the drive speed is zero or at least when no greater speed difference is present. If the required drive torque for the switching in is taken into account, the limit value for the switching in can be higher than the limit value for the switching off.
In an advantageous further development of the invention, the named drive apparatus forms the traction drive of a self-propelled work machine, with the drive part to be driven in common by the at least one electric motor and the hydraulic motor which can be switched in being able to be a wheel of the chassis or with a tracked chassis the drive pinion of the tracked chassis. In this respect, the at least one electric motor and the hydraulic motor connectable thereto can drive a single wheel in the sense of a one-wheel drive or can drive the plurality of wheels of a chassis axle in the sense of an axial drive, optionally via an interposed differential. Depending on the configuration, the at least one electric drive and the hydraulic motor which can be switched in can also drive a plurality of axles.
The use of such a drive comprising at least one electric motor and a hydraulic motor which can be switched in can in particular be of advantage for the traction drive of a dump truck or of a truck since here as a rule limited drive torques are required in normal drive operation and thus over a large portion of the operating time and the switching in of the hydraulic motor is only required when starting and thus in smaller operating time portions.
In an advantageous further development of the invention, provision can in this respect be made that the at least one hydraulic motor can be supplied, where required, from the pump or pressure source of the tilting mechanism of the dump body of the dump truck. The tilting mechanism of the dump body is typically only actuated on a standstill of the dump truck, while it is not required in drive operation. The pump or pressure source of the tilting mechanism then not required for the starting of the dump truck can in this respect be used to feed the hydraulic motor of the traction drive. For this purpose, the hydraulic motor can be connected to the pressure circuit of the tilting mechanism via a suitable connection line, preferably while interposing a valve. The pump of the hydraulic tilting mechanism can be driven by the aforesaid internal combustion engine.
Alternatively to such a design of the drive apparatus as a traction drive, the output of the drive apparatus can also be connected to a hoisting drum, for example to the hoisting drum of a crane or of another piece of lifting gear, by means of which a hoist rope or a guying rope of a crane can be adjusted.
In a further development of the invention, the electric motor can be dimensioned too small for the required starting torque or for the maximum drive torque of the work machine and can ideally be designed for the already started work operation or drive operation, in particular for the operating range of the most frequently used speed and/or of the most frequently used torque. The electric motor can in particular have a maximum starting torque and/or a maximum drive torque which is smaller than the maximum starting torque or maximum drive torque required in accordance with its intended purpose for the traction drive unit and/or main drive unit of the work machine, with, on the other hand, the at least one electric motor and the hydraulic motor which can be switched in being configured or dimensioned such that the SUM of the maximum starting torques and/or drive torques of the electric motor and of the hydraulic motor is at least as large as the aforesaid starting torque or drive torque which the traction drive or the main drive unit of the work machine requires. Not only weight can be saved by the under-dimensioning of the electric motor, the costs of the electric motor are also reduced.
The present invention will be explained in more detail in the following with reference to a preferred embodiment and to associated figures.